Ultrasonic applications in science and industry often require that ultrasound be transmitted efficiently from an electroacoustic material (such as a piezoelectric disc) of characteristic impedance Z.sub.1 to a nearby medium (such as a fluid, and particularly a cryogenic liquid) which is of relatively low characteristic acoustic impedance Z.sub.4. If the piezoelectric material is a lead zirconate titanate compound such as that designated PZT-5, and if the nearby fluid is water, then the mismatch ratio of their longitudinal wave characteristic impedances is R.sub.m =Z.sub.1 /Z.sub.4 =33.4/1.5=22.3. If R.sub.m could be halved, in effect, then energy transmission from transducer to fluid could be increased by 2.6 dB in this example.
To increase the efficiency of energy transfer, it is customary to interpose one or more layers of materials of impedances intermediate between Z.sub.1 and Z.sub.4. For example, matching at the frequency f.sub.1 or its odd harmonics can be achieved by using a material of impedance Z.sub.2 =.sqroot.Z.sub.1 Z.sub.4 (the geometric mean) which is a quarter-wave (.lambda./4) thick. To achieve matching over a broader band of frequencies, two or more quarter-wave layers can be bonded together as described by Desilets, Fraser and Kino, IEEE Trans. Sonics and Ultras. SU-25 (3) 115-125 (May 1978); de Belleval and Lecuru, 1978 Ultras. Symp. Proc., IEEE Cat. #78CH1344-1SU, pp. 122-125 (1978); Souquet, Defranould and Desbois, IEEE Trans. Sonics and Ultras. SU-26 (2) 75-81 (March 1979). These references indicate that appropriate selection of the impedance of front and backing members for the transducer improves the efficiency of energy transmission into a fluid such as water, and/or improves (shortens) the response time for impulsive excitation, as opposed to continuous wave (cw) excitation. The references state or imply that best results are obtained if the matching members bear a monotonically decreasing impedance relationship from the transducer towards the fluid. Souquet et al also indicate the advantage of mismatching the backing impedance to avoid wasting energy in the backing; essential to this mismatching was the use of a relatively low-impedance solid quarterwave layer, for which they selected epoxy, backed in turn by a material of equal or higher impedance than the transducer element, such as steel or tungsten-loaded epoxy. Other references helpful to an understanding of impedance matching are Communication Engineering, W. L. Everett and G. E. Anner, 3rd Ed., McGraw-Hill, 1956 (p. 403) and Waves in Layered Media, L. M. Brekhovskikh, Academic Press, N. Y., 1960 (p. 139).
One preferred broadband impedance-matching procedure is to use the binomial transformer approach, as illustrated by Desilets et al, their eq. (15).
Another impedance-matching method includes interposing a material of continuously graded impedance (Goldman, Ultrasonic Technology, Reinhold, New York (1962), pp. 75-78). Another method of matching uses a composite matcher consisting of layers whose impedances are stepwise-graded such that their combination of characteristic impedances Z.sub.2a, Z.sub.2b, . . . and thicknesses x.sub.a, x.sub.b, . . . transforms the relatively low fluid impedance Z.sub.4 to a higher value more nearly comparable, or, ideally, equal to, the transducer's impedance Z.sub.1 (Fry and Dunn, J. Acoust. Soc. Amer. 34, p. 188, 1962). A matcher's "effective impedance" Z.sub.eff may be defined as that impedance which would produce the same degree of improvement in energy transmission as obtained by the actual matcher. If the matcher is homogeneous, Z.sub.eff =Z.sub.2. We shall limit the scope of this invention to cases where Z.sub.1 &lt;Z.sub.eff &lt;Z.sub.4, whether the matcher is homogeneous or not (e.g., layered).
Still another matching method is the following. A single homogeneous material of intermediate impedance and of length longer than the pulse width (nonresonant matcher) may be used as described by Lynnworth in IEEE Trans. Sonics and Ultrasonics SU-12 (2) 37-48 (June 1965). Using a length for the backing and/or matching members longer than the pulse width, or sometimes, merely longer than 1/2 or 1 cycle of the pulse, prevents backing or matcher reverberations from interfering with the sought portion of the received pulse.
In the Lynnworth 1965 reference, it is shown that a thin metal shim, "floating" upon water, did not significantly attenuate an ultrasonic beam propagating from the water into the air above it. In 1974 (Panametrics Final Report, Contract N00014-73-C-0023, dated May 31, 1974, edited by Lynnworth and Papadakis, Page 7-59) several illustrations appeared, showing the transmissive character of a thin shim or sheet at oblique incidents. However, in none of these illustrations does the thin shim serve as a sealant, nor, when proposed as a wear-resistant or temperature-resistant part of an angle beam transducer (cited report, FIG. 7-11-1 (d)), does the thin shim enclose an impedance matching medium between a transducer and a fluid under test. In that cited figure, part (c) shows a plastic wedge transducer coupled to a stainless steel tube of 1.6 mm wall thickness. At the cited test frequency of 1 MHz, the tube wall provides a thickness compared to longitudinal wavelength of about 1/4, i.e., the tube wall is much thicker than the sealant thickness of the present invention. The tube wall is part of the illustrated flow cell or spoolpiece, not part of the external wedge-type transducer assembly. In 1978 Lynnworth, Pedersen, Seger and Bradshaw (Advanced Technology Fuel Mass Flowmeter, USARTL-TR-78-45, October 1978) utilized a thin metal shim such as 50 .mu.m (0.002 inches) stainless steel to support and seal piezoelectric transducers for use in aviation fuels or fuel substitutes such as Stoddard solvent. In the latter work, however, the shim was epoxied directly against the transducer element, with no intermediate impedance-matching member.
A polyvinylidene (PVF.sub.2) polymer microprobe developed by Wilson, Tancrell and Callerame was reported in the IEEE 1979 Ultrasonics Symposium Proceedings, pp. 506-510, in which the PVF.sub.2 was completely encased in a metal cylinder to shield it from spurious electromagnetic interference and to protect the polymer from possible corrosion by external fluids. However, this polymer transducer, only 30 .mu.m thick, was coupled directly to the front sealant stainless steel shim (of 25 .mu.m thickness) by a combination of pressure and oil. The pressure was provided by a silver-loaded rubber backing, the silver providing electrical conductivity. No impedance-matching layer was used.
For matching into a fluid of very low relative impedance, such as air, the intermediate material has sometimes been formulated by combining microballoons in epoxy, or by using porous grades of graphite.
Sometimes, the stability of a matched construction can be even more important than the efficiency of the match. For example, in immersed nondestructive testing where there is a requirement to monitor for long times the amplitude of a flaw echo, changes in the matching could be interpreted erroneously as changes in the flaw severity. Thus, to be reliable in critical applications, a quarterwave matcher must retain its bond integrity, dimensions, sound speed and density. If it unbonds, deforms in use, swells due to absorbed moisture, or changes in sound speed or attenuation, its ability to match will be modified and interpretive errors may ensue.
Since immersed ultrasonic testing is common, maintaining the stability and reproducibility of matching members in the presence of water has received the attention of transducer designers for many years.
In the field of industrial flow measurement by ultrasound, particularly flow of cryogenic fluids such as liquid nitrogen at -200.degree. C., or liquid oxygen or liquid methane at comparable temperatures, or liquid helium at even lower temperature, the problems of matching and sealing are further aggravated by differential thermal expansion or contraction. Materials compatibility is also important; for example, only a limited number of materials may be exposed to liquid oxygen without the risk of catastrophic reaction.
For high-temperature applications, transducer structures are needed which can be assembled at a temperature far enough below the transducer's Curie point to avoid depoling, yet which function reliably at the elevated temperature application. One procedure is an indium bonding method reported in Ultrasonic Transducer Materials, Mattiat, p. 162, Plenum Press (1971), wherein In and Au layers combine at room temperature but form a bond usable to 600.degree. C. Sometimes less expensive bonds can be formed by thermal diffusion. In such applications sealing layers may be hermetically sealed by electron beam welding to a tubular metal housing. The electron beam welded seal may be accomplished by sandwiching the sealant shim between a cylindrical tube wall and a washer. This combination may be fused together at their common perimeter. The washer may then be machined away by grinding or turning, followed by lapping, so that the resultant outermost face is flat enough not to trap air or other gas, in an immersion application. In cryogenic applications it is often unnecessary to remove the washer. The washer provides some mechanical protection against impacts encountered in handling and/or installation. However, the washer should be eliminated if it introduces undesirable diffraction effects.
The conventional impedance matching theory indicates a sequence of layered materials of monotonically decreasing characteristic impedance between transducer and low-impedance medium. Unfortunately, this approach restricts the sealing material to a low impedance, which normally means a relatively soft or inelastic material like plastic, urethane or epoxy. Such materials may be excellent acoustically, but are neither impervious nor mechanically stable, especially in severe environments. On the contrary, the materials which are impervious and mechanically durable and stable tend to be elastic and of relatively high impedance, like steel, and may have impedances even higher than that of the transducer, and certainly, in such cases, higher than that of the matcher by a factor of two or larger.